Efficient suppression of the signal from flowing blood is especially important for cardiovascular MRI, where anatomic structures and pathologic tissues of interest are close to blood, and flow artifacts may cause severe problems for diagnostic interpretation. Blood-suppressed (black-blood) imaging is currently based on two well-known techniques: in-flow saturation (see Edelman, R. R. et al., “Extracranial carotid arteries: evaluation with “black blood” MR angiography,” Radiology 1990, 177:45-50; and Steinman, D. A. et al., “On the nature and reduction of plaque mimicking flow artifacts in black blood MRI of the carotid bifurcation,” Magnetic Resonance Medicine 1998, 39:635-641), and double inversion-recovery (DIR) (see Edelman, R. R. et al., “Fast selective black blood MR imaging,” Radiology 1991, 181:655-660; and Simonetti, O. P. et al., “Black blood T2-weighted inversion recovery MR imaging of the heart,” Radiology 1996, 199:49-57). The method of in-flow saturation usually does not enable complete elimination of the signal from flowing blood and may be ineffective in applications that require clear visualization of the interface between vessel wall and lumen, such as high-resolution imaging of atherosclerotic plaque. The DIR technique is known, to date, as one of the more effective black-blood imaging modalities. The principle of DIR is that the signal from all spins within a transmit coil volume is inverted by a non-selective 180° pulse, which is followed by a slice-selective 180° pulse to restore the magnetization of a slice that is about to be imaged. After a properly chosen inversion time (TI), the magnetization of inflowing blood achieves a zero-crossing point, so that the observed signal does not contain a contribution from blood. Thus, DIR provides two general advantages: (1) it has the potential to completely eliminate the signal from flowing blood; and, (2) it has minimal sensitivity to the blood flow rate, which, in fact, should guarantee only the outflow from a relatively thin imaged slice. However, DIR, as used in this manner, is essentially applicable to imaging only a single slice per repetition time (TR), and therefore, a long scan time is required if several slices need to be imaged using the DIR method.
To improve the time efficiency of DIR, two multi-slice acquisition strategies were recently proposed. Song et al., as reported in “Multislice double inversion pulse sequence for efficient black-blood MRI,” Magnetic Resonance Medicine 2002, 47:616-620, developed a double-slice technique that uses one non-selective and two consecutive slice-selective inversions. Following the TI period, fast spin-echo (FSE) readouts are then applied consecutively to the corresponding slices to acquire the signals for imaging the slices. With this approach, only the signal for imaging one slice can be acquired at the moment of zeroing blood magnetization, while the acquisition of the signal for imaging the second slice is delayed from the zero-crossing point of the blood magnetization by the duration of the readout process. Any further increase in the number of slices acquired within one repetition time (TR) requires a shorter readout sequence to be used to minimize the effect of flowing blood. A five-slice extension of this technique was recently demonstrated with spiral readout by Song et al. (see “Highly efficient double-inversion spiral technique for coronary vessel wall imaging,” Proceedings of the 10th Annual Meeting of ISMRM, Honolulu, 2002, p. 1566). However, the use of a relatively long FSE readout sequence, which is preferable in most applications due to image quality and tissue contrast, may preclude an increase in the number of slices per TR. Parker et al. (see “Improved efficiency in double-inversion fast spin-echo imaging,” Magnetic Resonance Medicine 2002, 47:1017-1021) proposed an alternative method that enables acquisition of each slice at the exact zero-crossing point. In this technique, the slices are acquired sequentially within the TR, while TI is reduced. This method results in several zero-crossing points per TR, each for the corresponding slice. Similar to the method of Song et al., a preparative module in the method of Parker et al. consists of a non-selective inversion followed by a train of slice-selective inversions applied successively to the slices to be imaged within one TR. The method was demonstrated in two- and four-slice variants. The four-slice technique, however, was implemented with a doubled TR that resulted in the same time efficiency as the two-slice procedure.
A common problem of the above-noted multi-slice DIR techniques is the construction of a preparative module, which includes a train of slice-selective inversions, creating unequal conditions for the evolution of magnetization of different slices, since the delay between non-selective inversion and slice-selective re-inversion depends on the actual slice number. Furthermore, the restrictions on specific absorption rate (SAR) also may preclude a further increase of slice quantity. It is especially critical for the method of Parker et al., in which the number of slice-selective inversion pulses per TR is equal to the square of the number of slices, where for example, 16 inversions should be applied for four-slice imaging.
To overcome this problem, it would be desirable to develop an alternative multi-slice DIR technique that is based on the simultaneous re-inversion of a group of slices. Such a technique should provide the required time efficiency without the problems of the prior art, while enabling the signal for imaging each successive slice to be acquired at the time the contribution to the signal from flowing blood is substantially zero or approaching zero.